1. Field of the Invention
This invention relates to the field of apparatus for applying joint tape and adhesive over the joints between wallboard sections.
2. Description of Prior Art
In what is believed to be the only automatic wallboard joint taper presently employed on a large scale, traction wheels (drive rollers) which roll along the wallboard are used to drive a drum on which a cable is wound. The cable is attached to a piston in the cylinder or reservoir wherein the adhesive (called cement or "mud") is stored. When the piston is at the top of its stroke it can move no further. Thus, flow of adhesive rapidly ceases. When this happens, it is a natural human tendency for the operator to push harder on the traction wheels-the thought being that more adhesive will then flow. Instead, however, what can happen (in the event of malfunction of the below-described release mechanism) is that the cable breaks. In such event, there results a time-consuming disassembly and repair job.
Even when the cable doesn't break, it is highly undesirable for the piston to fully reach the top of its stroke without being at least partially disconnected. This is because the traction wheels are then suddenly braked, causing a jerking stop and possible dislodgment of the already-applied tape.
During approximately the past ten or twenty years, at least two approaches have been made toward solving the stated problems. One proposed solution--the one presently in widespread commercial use on the above-mentioned automatic joint taper--is to provide a pin element disposed to be engaged by the piston and to operate a lever mechanism adapted to fully release the drive to the cable drum as the piston reaches the top region of its stroke. After the drive is thus fully released or disengaged, automatically, the operator uses his thumb to push on a disc to thus shift a second pin (connected to the disc) which is coaxial with the drum. Such second pin maintains the drive released, despite the presence of a return spring bias, so long as the second pin remains thus shifted. To maintain the second pin thus shifted, and to simultaneously close a valve which prevents discharge of adhesive during reservoir filling, the operator pivots the valve handle over the indicated disc. Stated otherwise, the valve-closing element performs the second function of maintaining the disc and second pin shifted (so that the cable drum drive stays disengaged). Because the drum drive stays disengaged, there is no throwing of adhesive during reseervoir filling.
The described drive-release mechanism is disadvantageous in several respects, which are here stated but not necessarily in order of importance. Firstly, it uses exposed, external lever and other elements which may become broken, gummed-up or otherwise disabled. When this occurs, the cable may break and the tool may come to a jerking stop--with attendant harmful results as described above. Secondly, the described release mechanism uses a pin which extends inwardly through the end wall of the cylinder and thus is exposed to adhesive, creating problems of operation and maintenance. Thirdly, the operator often does not hear the noise made by the release mechanism and therefore is not cognizant of the fact that stroke-end has been reached, so he goes on taping (for a short period) in the absence of any adhesive to secure the tape to the wallboard. Fourthly, the means for maintaining the mechanism released requires that the thumb and at least one other finger of the operator be used immediately prior to the filling operation described below (one digit being needed to press on the disc, another to pivot the valve handle). Fifthly, denting of the thin-walled barrel may cause the piston to jam long before it reaches the end of its stroke, with resultant probability of cable breakage.
A second approach toward solution of the problems of preventing cable breakage, and preventing tool jerking when the piston reaches the top of its stroke, is simple but impractical. This is to make the cylinder of transparent material so that the operator can watch and see the position of the piston. However, the operator is normally watching the joint, not the cylinder, and either cannot or will not--in a sufficently high percentage of cases--discern when the stroke-top is being closely approached.
Referring next to the filling of the reservoir with adhesive, this is necessarily accompanied by release of the cable-drum drive or at least two bad results occur. The traction wheels (drive rollers), sprockets and chain are often covered with adhesive and if allowed to spin rapidly--as the result of pushing of the piston to the bottom of its stroke while the reservoir chamber is filled--would cause adhesive to be thrown centrifugally onto the floor and walls. Secondly, connection of the wheels and sprockets during filling would increase the resistance to injection of adhesive into the reservoir. Thirdly, rapidly spinning sprockets and toothed traction wheels would create an accident hazard.
As above stated, the above-indicated prior-art automatic taper does maintain drive disengagement during filling, but does so in an awkward manner requiring not only large external elements but also the use (immediately before commencement of filling) of at least two digits of the operator.
Proceeding next to other major characteristics of prior-art joint tapers, it is emphasized that (a) the tape must be cut at the end of each joint, and (b) such cutting must occur after tape-feeding stops. If, as happens occasionally even with highly experienced operators, the cutting operation is attempted while the tool is still moving, the tool will usually jam. Jamming occurs because the moving (still being fed) tape bunches up against the side of the cutting blade.
In the above-described joint-taping apparatus now used commercially to a large extent, such jamming often results in a substantial delay. This is because the cutting blade is relatively inaccessible, being only reachable after a time-consuming disassembly operation. Thus, it is important that the jammed cutting blade and adjacent regions be rendered accessible in a matter of seconds. Applicants have accomplished this, and have simultaneously solved a second problem present in prior-art joint tapers.
This second problem is that of applying only adhesive or "mud" (not tape) over a strip of tape already present on a joint. It is common to first cause a tape strip to adhere over a joint, by means of adhesive, then to let the adhesive dry, then to apply a second layer of adhesive, and then to "work" such second layer so that it blends into the wall. Previously, insofar as applicants are aware, such second layer has been applied (1) manually, or (2) by an automatic tool entirely separate from that used for first laying of the tape, or (3) by a special "follower" portion of a tape-laying tool. Manual operation, with only a knife, etc., has obvious disadvantages in regard to such factors as speed. The use of a separate tool requires additional costs, storage room, etc. The immediate laying of a second layer over the tape, before the first layer has dried, greatly increases the overall drying time. For this reason, and because the special follower portion is heavy and awkward to use, the third of the specified alternatives is also not satisfactory.